1. Field of the Invention
This invention is in the field of measuring the surface forces and mechanical properties of materials; and, specifically, to measurements where nanoNewton (nN) forces, and less than nanometer (nm) distances must be resolved. Forces range from 10.sup.-3 to 10.sup.-9 Newtons and distances range from 10.sup.-6 to 10.sup.-10 meters. In addition to surface imaging, instruments of this type can be used to measure surface and mechanical properties of solid materials and the data from these surface measurements may be used to determine properties of materials such as hardness, elasticity, adhesion, surface tension, surface energy, and surface forces such as Van der Waals forces.
b 2. The Prior Art
This invention combines the capabilities of three different prior art instruments: the surface force apparatus, the hardness tester or indenter, and the atomic force microscope.
The surface force apparatus is used to study surface forces and adhesion. It consists of two crossed mica cylinders with radii of curvature on the order of 1 cm. One cylinder is in a fixed position; the other cylinder is suspended on a cantilever. The relative position of the cylinders, and thus the cantilever deflection, is determined by multi-beam interferometry. Using Hooke's Law (F=-kz), the interaction force between the cylinders is determined from the cantilever deflection times its effective spring constant. The surface force may be studied as a function of the distance between the cylinders. This technique is capable of measuring the separation of two surfaces to 0.2 nm but its lateral resolution is poor due to a contact area on the order of 10-30 .mu.m.sup.2. The force resolution, typically 10 nN, depends on the effective spring constant of the cantilever.
The hardness tester (also called a micro or nano indenter, materials tester, or mechanical properties microprobe) generates curves of force versus penetration depth. Mechanical properties of materials, such as elasticity and hardness, may be determined from the loading curves. After the indenter contacts the sample, the load (force) applied to the indenter tip causes the tip to penetrate into the sample surface. The applied load is plotted as a function of the penetration depth. Penetration depth resolution is as high as 0.4 nm. Force resolution with commercially available instruments is currently 300 nN; however, 100 nN. For comparison, this invention offers a force resolution of 1 nN and a depth resolution of 0.02 nm.
The atomic force microscope is used to image the surface of a sample with nanometer spatial resolution. As the sample moves under a probe mounted on a cantilever beam, the deflection of the beam determines the applied force. Beam deflection is measured by an electron tunneling microscope, optical measurements, or capacitance measurements. Plotting the cantilever deflection as a function of the sample's x and y position generates a nanometer-scale image. In the past, the atomic force microscope was used only for imaging.